Differential timing transfer is used when there is a network interface with its own source clock and there is the need to transfer this clock over a core packet network (with its own independent network-wide reference clock) to another interface. This transfer is performed while each interface and the core network maintain their timing traceability to their independent source clocks.
Differential clocking, such as the Synchronous Residual Time Stamp (“SRTS”) method specified for Asynchronous Transfer Mode (“ATM”) networks, allows equipment at the edges of a packet network to use a clocking signal that is different and completely independent from the clocking signal being used in the core packet network. The Residual Time Stamp (“RTS”) measures the difference between the service clock received on a Time Division Multiplexing (“TDM”) interface and the network-wide reference clock. Stringent protocol requirements of SRTS require that the RTS value be specified using four bits and must be sent per eight cells using one bit in the ATM Adaptation Layer 1 (“AAL1”) header for every odd sequence numbered cell. The RTS is propagated to the remote end of the circuit in the AAL1 header of the ATM cell. The receiving end reconstructs the clock by adjusting the reference clock by the RTS value.
Adaptive clock recovery (“ACR”) is another technique used to distribute and recover clock from a packet network. The task of clock recovery is an “averaging” process that mitigates the deleterious effects of network impairments, such as random packet delay variation (“PDV”), and captures the average rate of transmission of the original bit stream. In adaptive clock recovery, because there is no network-wide reference clock available, the performance of ACR techniques are subject to packet network impairments and cannot be guaranteed. For high performance, adaptive clock recovery requires that the slave implement very sophisticated phase locked loops and highly expensive oscillators.
Synchronous Ethernet network synchronization is a recently developed technology used to extend the well-known concepts of TDM network synchronization into the domain of packet-based networks, which until now have been specified as asynchronous in nature. The timing standard for Synchronous Ethernet implementations has been defined by the Telecommunication Standardization Sector of the International Telecommunication Union (“ITU-T”), in recommendation G.8261, entitled “Timing and Synchronization Aspects in Packet Networks.” Synchronous Ethernet provides frequency distribution via the Ethernet physical layer. Synchronous Ethernet is essentially native Ethernet equipped with timing traceability at the physical layer.
For traditional Ethernet networks, the timing sources at each edge of the network cannot be synchronized to each other. Therefore, what is needed is a simple, accurate, and inexpensive system and method for providing differential timing transfer over Synchronous Ethernet networks.